A microwave high power amplifier employed for a digital mobile telephone or the like requires low distortion characteristics for avoiding adjacent channel interference. To improve the distortion characteristics, phase rotation and gain compression of input/output signals of a transistor used for the power amplifier should be suppressed.
In a prior art high power microwave power amplifier, in order to compensate phase rotation and gain compression, caused by the characteristics of a o n transistor used as an amplifying element, a diode, an FET, or a bipolar transistor is inserted in a stage before the transistor.
In addition, usually in a high power and high efficiency two-stage amplifier which requires excellent distortion characteristics, an element of the first stage is employed in a linear region, i.e., a region having low distortion, that is, having less gain compression and less phase rotation, and an element of the second stage improves output power and power added efficiency in a range where the standard of distortion is satisfied. In the prior art microwave power amplifier, in order that the element of the first stage have an increased linear region and the element of the second stage satisfy the standard of distortion, the sizes of the respective elements are made larger.
In the prior art microwave power amplifier, another element is inserted in a stage before the transistor in order to compensate the phase rotation and gain compression, so that the number of parts is increased, resulting in a cost increase.
In the prior art microwave power amplifier, the sizes of the elements are made larger for fabricating a high power two-stage amplifier which requires excellent distortion characteristics, so that the whole circuit becomes larger in size, resulting in a cost increase and difficulty in realizing high efficiency.